js: Convert static/js/message_live_update.js to ES6 module.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

static/js/user_events.js

@@ -4,6 +4,7 @@
 // server_events.js simple while breaking some circular
 // dependencies that existed when this code was in people.js.
 // (We should do bot updates here too.)
+const message_live_update = require("./message_live_update");
 const people = require("./people");
 const pm_list = require("./pm_list");
 const settings_config = require("./settings_config");